A healthy knee joint is able to withstand great forces that are exerted as the knee flexes and extends and supports the weight of the body. However, when the knee joint becomes diseased, damaged or is otherwise unable to withstand the forces required of that joint, it may become necessary to reconstruct or replace the knee joint. When replacement is necessary, the natural knee joint is replaced with a prosthetic knee joint. A typical knee joint prosthesis includes a femoral component and a tibial component. During a replacement knee surgery, portions of both the tibia and femur are typically resected to allow the placement of prosthetic tibial and femoral components, which are anchored to the respective bones.
Sometimes, a small percentage of patients who undergo a total knee replacement surgery suffer from infections in the knee joint at the surgical site. To alleviate the effects of the infection, a two-stage revision of the failed knee replacement is employed. First, the failed prosthesis must be surgically removed and the site debrided and cleansed extensively in order to rid the site of the infection. Before a new, permanent prosthesis can be placed in the old surgical site, the site must be free of infection. A temporary antibiotic-impregnated cement spacer may be used as part of the therapy to rid the site of infection. Disinfecting the site of infection may take between 6-8 weeks and up to 3-4 months in most circumstances. It is thus common for surgeons to replace the old prosthetic knee with a temporary implant, typically made of bone cement, during the 6-8 week period while the infection is cleared up and before the new prosthesis is surgically implanted. The second and final step requires a separate revision surgery to then replace the temporary implant with a permanent prosthetic implant.
In the past, surgeons have been left to their own devices when forming cement implants, including the use of negative molds. The process of making a negative mold consists of the surgeon creating a mold by inserting a portion of bone cement into a bowl or other mixing container and allowing the cement to nearly cure. Prior to complete curing of the bone cement, the surgeon inserts the articulating end of the femoral component into the bone cement to create a mold. Using that mold, the surgeon then applies an oil to the mold creating a barrier for separating the cement implant from the cement mold. After applying the oil, cement may be poured into the mold allowing it cure, after which the surgeon attaches the resulting bone cement implant onto the femur as a temporary replacement.
Other methods used in the past of forming temporary implants include surgeons creating the implant with their own hands or simply putting a block of cement between the tibia and the femur to act as a spacer. However, there are many problems associated with such methods and designs, namely increased surgical time due to the preparation and formation time needed for creating the implant. Particular problems associated with the block or spacer method include completely immobilizing the knee in an extended position, after surgery, for the entire 6-8 week period, which in turn leads to soft tissue damage and further complicates the revision surgery. Therefore, reproducing the knee joint using temporary implants that simulate the natural tibial and femoral components of the knee joint is much more desirable because it permits the patient to move his/her leg through a minimal range of motion. The range of motion, while limited, significantly increases the patient's comfort over the 6-8 week period allowing the patient to bend his/her knee for sitting in a chair or for riding in a car and also increases the ease of the revision surgery because the soft tissue has not been damaged to the same extent as when the knee is completely immobilized.
Attempts have been made in the prior art to provide alternatives to surgeons creating their own negative molds or even molding a temporary implant by hand, including the use of pre-made, disposable molds. Such attempts include several drawbacks, however. For example, there may be a need for many different sized molds to accommodate the differences in size from patient to patient. Existing molds are prone to overfilling and spillage, leading to wasted materials and a messy work area. Further, some molds require a surgeon cut the mold to remove it from the implant once cured. This scoring separation can be quite cumbersome to achieve, can result in small, contaminating particles or pieces of the mold or molded prosthesis being strewn about, as well as increasing the likelihood that the molded prosthesis itself is inadvertently cut or damaged in the process. Additionally, when a mold is filled with a curable material to make the prosthesis, the pressure inside the mold can cause the mold itself (and thus the resulting prosthesis) to deform.
In view of these drawbacks, it is desirable to provide orthopedic prosthesis molds and methods of use thereof that safeguard against overfilling, spillage and deformation, are easily separable to reveal the molded prosthesis, and provide an accurate, selectable range of molded prosthesis sizes.